Television systems are known in which an interlaced video signal is converted to a non-interlaced or "progressively scanned" form in which the number of horizontal lines displayed in a field is doubled. Advantageously, such systems reduce the visibility of the line structure of displayed images.
Since doubling the number of displayed lines requires more lines than are actually transmitted, there have been a number of proposals for obtaining the required "additional" lines. An example of a system in which the required "extra" lines for display are obtained by repeating lines of a received signal is described by R. A. Dischert in U.S. Pat. No. 4,415,931 entitled TELEVISION DISPLAY WITH DOUBLED HORIZONTAL LINES which issued Nov. 15, 1983. An example of a system in which the "extra" or interstitial lines are obtained by interpolation of adjacent vertical lines of the received signal is described by K. H. Powers in U.S. Pat. No. 4,400,719 entitled TELEVISION DISPLAY SYSTEM WITH REDUCED LINE-SCAN ARTIFACTS which issued Aug. 23, 1983.
The above mentioned systems described arrangements in which extra lines for display are derived from a currently received field of a video input signal. This form of progressive scan conversion is commonly known as "intra-field" or "line" conversion and has an advantage in that there are no visible artifacts produced for images containing field-to-field motion. However, there is a disadvantage in that the vertical resolution of displayed images is not improved and may be degraded, particularly where vertical interpolation is employed, and this tends to "soften" displayed images.
It has been widely recognized that the added lines needed for a progressive scan display can be obtained from a previous field rather than from a currently received field. Such systems are known generally as "field" or "inter-field" progressive scan systems and double the number of displayed lines by interleaving lines of a currently received field with lines of a previously received field. An advantage of "field" progressive scan processing is that still images are produced with the full vertical resolution of an originally scanned frame of video.
An example of a "field progressive scan" system is described by Okada et al. in U.S. Pat. No. 4,426,661 entitled TELEVISION RECIEVER INCLUDING A CIRCUIT FOR DOUBLING LINE SCANNING FREQUENCY which issued Jan. 17, 1984. See also, U.K. Application GB 2,114,848A of Achiha et al. published Aug. 24, 1983 and entitled COLOR TELEVISION SIGNAL LINE DOUBLING CIRCUIT. Unfortunately, field progressive scan systems suffer from a problem in that if field-to-field motion exists in a scene the displayed images will be blurred.
A further problem with progressive scan processors of the type in which extra lines are derived from a previous field is that a relatively substantial amount of memory is required for storing (delaying) the lines of the previous field.
Systems have been proposed in which the problem of vertical resolution characteristic of line-progressive scan systems and the problem of motion blur characteristic of field progressive scan systems has been approached by making the systems "motion adaptive". In motion adaptive systems a motion detector is used to switch between the two basic types of processors as a function of motion. For example, when the incoming video signal represents a still image the signal is processed by a field type processor which generates extra lines for display by interleaving the currently received lines with lines of a previous field. Conversely, when the incoming video signal represents a moving image, the extra lines for display are obtained by interpolation (or repeating) lines of the currently received field. For images that are not still and are not in full motion it is customary to "blend" or mix the outputs of line and field type processors in proportion to the magnitude of the motion.
A first example of a "motion adaptive" progressive scan converter is described by Casey in U.S. Pat. No. 4,598,309 entitled TELEVISION RECEIVER THAT INCLUDES A FRAME STORE USING NON-INTERLACED SCANNING FORMAT WITH MOTION COMPENSATION which issued Jul. 1, 1986. In an embodiment of the Casey system, an interlaced signal is converted to non-interlaced form for display. When no picture change (motion) occurs, the picture information of the first and second fields of the input signal are displayed in alternate scan lines of the display device to form a picture frame during one vertical scanning interval. A motion indicating signal, indicative that a picture change occurs in a section of the picture frame, is generated by use of a frame comb filter to detect inter-frame picture variations. The picture change indicative (motion) signal causes the displayed picture information in that section of the picture frame to be derived from one field of the input video signal.
Advantageously, the motion indicating signal in the Casey apparatus is a multi-bit signal having an amplitude representative not only of the presence of motion, but also representative of the relative degree or amplitude of the motion. The selection of interfield or frame processing is provided by a so-called "soft" switch in proportion to the motion signal amplitude to provide a "blended" output signal having field delayed and line-averaged components in proportion to the motion signal amplitude. In other words, rather than merely switching from field to line processing upon detection of motion, the Case apparatus provides a smooth transition from one mode to another by blending the field and line processed video signals in accordance with the motion signal amplitude.
A second example of motion adaptive processing is described by Fling in U.S. Pat. No. 4,639,783 entitled VIDEO SIGNAL FIELD/FRAME STORAGE SYSTEM which issued Jan. 27, 1987. In an embodiment of the Fling system, a multi-bit motion signal is generated by frame comb filtering of the video signal and used to control a "soft" or "proportional" switch much as in the aforementioned Casey apparatus. An advantageous feature of an embodiment of the Fling system concerns the use of a frame memory for obtaining both field delayed and frame delayed signals by means of a multiplexing technique. Specifically, the frame memory is operated in a serial mode at twice the video signal sample rate. Video samples are inputted to the memory on alternate states and alternate samples recovered from the memory are routed back to the vacant memory states. The result is that every other sample in the memory is field delayed and the in-between samples, having gone twice through the memory are frame delayed and so both field delayed and frame delayed samples are available at the memory output. By this means, a frame memory having no field tap thus serves the functions of a pair of cascaded field memories which otherwise would be required to provide a field tap and a frame delayed signal.
Motion adaptation is also used in video signal processing for purposes other than progressive scan processing. Wargo et al., for example describe a motion adaptive comb filtering system for separating a luminance signal Y from a composite video input signal in U.S. Pat. No. 4,716,462 entitled MOTION ADAPTIVE TELEVISION SIGNAL PROCESSING SYSTEM which issued Dec. 29, 1987. In an embodiment of their invention, a motion detector supplies a multi-bit motion signal to a soft switch for selection of line comb and frame comb filtered signals as a function of motion. The line comb filter is used when substantial motion exists, the frame comb filter is selected when essentially no motion exists and the line and frame comb filtered signals are "blended" in proportion to a non-linear control signal K derived from the motion signal for intermediate values of motion.
As a second example, motion adaptation may be used for video noise reduction as described, for example, by Fling in U.S. Pat. No. 4,639,784 entitled VIDEO SIGNAL RECURSIVE FILTER ADAPTIVELY CONTROLLED RESPONSIVE TO THE HISTOR OF IMAGE MOTION which issued Jan. 27, 1987. In an embodiment of the Fling system, a recursive filter for effecting noise reduction of video signals, sums current and delayed signals and includes a motion detector for detecting inter-frame image motion. Signal from the motion detector is stored to provide a history of image motion. A decoder coupled to the motion detector and also responsive to the stored motion history develops control sequences for scaling the signal contributions which form the recursive filter sums. In a specific implementation of the Fling apparatus, a frame delay is imparted to a one-bit motion signal using the same frame memory as for the active video signal being filtered. This is done by accumulating motion bits for one line interval and inserting the accumulated bits in the horizontal blanking interval of the next line stored in the frame memory with the active video bits being stored only in the active video portion of the line.